I. Field of the Invention
The present invention relates generally to a method of and apparatus for effecting handoff between different cellular communications systems.
II. Description of the Related Art
The so-called code division multiple access (CDMA) modulation technique is but one of several techniques for facilitating communications in which a large number of system users are present. Although other techniques, such as time division multiple access (TDMA), frequency division multiple access (FDMA) and AM modulation schemes such as amplitude companded single sideband (ACSSB) are also available, CDMA has significant advantages over these other modulation techniques. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “Spread Spectrum Multiple Access Communication System Using Satellite Or Terrestrial Repeaters”, assigned to the present assignee, the disclosure of which is incorporated herein by reference.
In U.S. Pat. No. 4,901,307 a multiple access technique is described in which a large number of mobile telephone system users, each having a transceiver, communicate through satellite repeaters or terrestrial base stations (also known as cell base stations, or cell-sites) using code division multiple access (CDMA) spread spectrum communication signals. In using CDMA communications, the frequency spectrum can be reused multiple times thus permitting an increase in system user capacity. The use of CDMA techniques results in a much higher spectral efficiency than can be achieved using other multiple access techniques.
In conventional cellular telephone systems the available frequency band is divided into channels typically 30 KHz in bandwidth while analog FM modulation techniques are used. The system service area is divided geographically into cells of varying size. The available frequency channels are divided into sets with each set usually containing an equal number of channels. The frequency sets are assigned to cells in such a way as to minimize the possibility of co-channel interference. For example, consider a system in which there are seven frequency sets and the cells are equal size hexagons. A frequency set used in one cell will not be used in the six nearest or surrounding neighbors of that cell. Furthermore, the frequency set in one cell will not be used in the twelve next nearest neighbors of that cell.
In conventional cellular systems, the handoff scheme implemented is intended to allow a call or other type of connection (i.e., data link) to continue when a mobile station crosses the boundary between two cells. The handoff from one cell to another is initiated when the receiver in the cell base station handling the call or connection notices that the received signal strength from the mobile station falls below a predetermined threshold value. A low signal strength indication implies that the mobile station must be near the cell border. When the signal level falls below the predetermined threshold value, the base station asks the system controller to determine whether a neighboring base station receives the mobile station signal with better signal strength than the current base station.
The system controller in response to the current base station inquiry sends messages to the neighboring base stations with a handoff request. The base stations neighboring the current base station employ special scanning receivers which look for the signal from the mobile station on the specified channel. Should one of the neighboring base stations report an adequate signal level to the system controller, then a handoff will be attempted.
Handoff is then initiated when an idle channel from the channel set used in the new base station is selected. A control message is sent to the mobile station commanding it to switch from the current channel to the new channel. At the same time, the system controller switches the call from the first base station to the second base station.
In the conventional system a call will be discontinued if the handoff to the new base station is unsuccessful. There are many reasons that a failure in handoff may occur. Handoff can fail if there is no idle channel available in the neighboring cell for communicating the call. Handoff can also fail if another base station reports hearing the mobile station in question, when in fact this base station actually hears a different mobile station using the same channel in a completely different cell. This reporting error will result in the call being switched to a wrong cell, typically one in which signal strength is insufficient to maintain communications. Furthermore should the mobile station fail to hear the command to switch channels, the handoff will fail. Actual operating experience indicates that handoff failures occur frequently which questions the reliability of the system.
Another common problem in the conventional telephone system occurs when the mobile station is near the border between two cells. In this situation the signal level tends to fluctuate at both base stations. This signal level fluctuation results in a “ping-ponging” situation in which repeated requests are made to hand the call back and forth between the two base stations. Such additional unnecessary handoff requests increase the possibility of the mobile station incorrectly hearing the channel switch command or failing to hear the command at all. Furthermore, the ping-ponging situation raises the possibility that the call will be discontinued if it is inadvertently transferred to a cell in which all channels are currently in use and thus unavailable for accepting the handoff.
In U.S. Pat. No. 5,101,501, entitled “Method And System For Providing A Soft Handoff In Communications In A CDMA Cellular Telephone System”, assigned to the present assignee, the disclosure of which is incorporated herein by reference, a method and system are disclosed for providing communication with the mobile station through more than one cell base station during the handoff. In this environment communication within the cellular system is uninterrupted by the eventual handoff from the base station corresponding to the cell from which the mobile station is exiting to the base station corresponding to the cell to which the mobile station is entering. This type of handoff may be considered as a “soft” handoff in communications between cell base stations with the mobile wherein two or more base station or sectors of base station transmit concurrently to the mobile station. The use of such “soft” handoff techniques has been found to substantially reduce the incidence of ping-ponging situations in which repeated handoff requests are made between a pair of base stations.
An improved soft handoff technique is disclosed within U.S. Pat. No. 5,267,261, entitled “Mobile Station Assisted Soft Handoff In A CDMA Cellular Communications System”, assigned to the present assignee, the disclosure of which is incorporated herein by reference. The soft handoff technique is improved by measuring at the mobile station the strength of “pilot” signals transmitted by each base station within the system. These pilot strength measurements are of assistance in the soft handoff process by facilitating identification of viable base station handoff candidates.
The improved soft handoff technique prescribes that the mobile station monitors the signal strength of pilots from neighboring base stations. When the measured signal strength exceeds a given threshold, the mobile station sends a signal strength message to a system controller via the base station through which the mobile station is communicating. Command messages from the system controller to a new base station and to the mobile station establish contemporaneous communication through the new and current base stations. When the mobile station detects that signal strength of a pilot corresponding to at least one of the base stations through which the mobile station is communicating has fallen below a predetermined level, the mobile station reports the measured signal strength indicative of the corresponding base station to the system controller via the base stations through which it is communicating. Command messages from the system controller to the identified base station and the mobile station terminates communication through the corresponding base station while communications through the other base station or base stations continue.
Although the foregoing techniques are well suited to call transfers between cells in the same cellular system, a more difficult situation is presented by movement of the mobile station into a cell serviced by a base station from another cellular system. One complicating factor in such “intersystem” handoffs is that the neighboring cellular system often has dissimilar characteristics. For example, adjacent cellular systems will often operate at different frequencies, and may maintain different levels of base station output power or pilot strength. These differences effectively preclude the mobile station from performing the pilot strength comparisons and the like contemplated by existing mobile-assisted soft handoff techniques.
When resources are not available to conduct soft intersystem handoffs, the timing of the handoff of a call or connection from one system to another becomes critical if uninterrupted service is to be maintained. That is, the intersystem handoff must be executed at the time most likely to result in successful transfer of the call or connection between systems. In such a handoff, referred to herein as a hard handoff, communication between the mobile station and one system must cease before communication between the mobile station and the other system can begin. It follows that the handoff should be attempted only when, for example:                (i) an idle channel is available in the new cell,        (ii) the mobile station is actually within range of the new cell base station, but before it loses contact with the current cell base station, and        (iii) the mobile station is in a position at which it is assured of receiving the command to switch channels.        
Ideally, each such hard intersystem handoff will be conducted in a manner which minimizes the potential for “ping-ponging” handoff requests between the base stations of different systems. However, this is made difficult as a result of the failure of existing handoff procedures to identify when, and through which base stations, the mobile station should supplied with new frequency and channel information and instructed to transfer the existing call or connection.
These and other shortcomings of existing intersystem handoff techniques impair the quality of cellular communications, and may be expected to further degrade performance as competing cellular systems continue to proliferate. Accordingly, there is a resulting need for an intersystem handoff technique capable of reliably directing the handoff of a call or connection between the base stations of different cellular communication systems.
U.S. Pat. No. 5,697,055, entitled “Mobile Station Assisted Soft Handoff In A CDMA Cellular Communications System”, assigned to the present assignee, the disclosure of which is incorporated herein by reference, describes a method and system for performing an intersystem handoff of communication with a mobile station between base stations of first and second cellular systems. At the mobile station, a quantifiable parameter of a signal transmitted by a second base station of the second system is measured. When the measured value of the quantifiable parameter passes through a first predetermined level, the mobile station communicates a signal quality message via a first base station of the first system to a first mobile switching control station.
A channel request message is then communicated from the first mobile switching control station to a second mobile switching control station within the second system. At the second base station, a quantifiable parameter of the signal received from the mobile station is also measured. The second base station establishes communication with the mobile station when the measured value of the quantifiable parameter passes through a predetermined level. Alternatively, the signal strength of a first pilot signal transmitted by the first base station is measured at the mobile station. A handoff request message is then sent to the second base station when the measured signal strength of the first pilot signal becomes less than a second predetermined level, thereby mobile station communication to be established. The provision of a voice link between the mobile switching control stations allows for the forwarding of an existing connection between the first and second cellular systems, and enables the performance of soft intersystem handoffs.
While this arrangement works well for situations where both systems are CDMA based and therefore both capable of performing soft handoff, there remains the problem of how to handle inter-system handoff where one or more of the systems is unable to perform such a handoff. For example, the so-called GSM standard has no mechanism for a soft handoff. There is, therefore, a problem in handing off a call using the air interface from a CDMA network to a GSM network. Furthermore, GSM authentication cannot be done because the CDMA 2000 mechanisms cannot transfer the data required to do GSM authentication. Encryption in GSM is different than the encryption in CDMA 2000.
One way of dealing with this problem would be to modify GSM to enable it to effect handoff to a non-GSM system, e.g. a CDMA system. However, GSM has been established for a long time now, relatively speaking, and operators will be reluctant to make expensive modifications to existing equipment in order to accommodate a neighbouring incompatible system. If new messages are added to the air interface in support of dual-mode mobile stations, then modifications must be made to support these new messages. Plainly, this is undesirable form the perspective of the operator.
Another problem with handing off between a CDMA system and a GSM system is that CDMA and GSM authentication use two different methods and keys. The authentication methods in GSM and CDMA 1X are basically the same, but the keys have different sizes. CDMA 1X has additional procedures such as unique challenge and count methods, which respectively prevent channel hijacking and replay attacks.